On the Mac they are in the Resources folder inside the app package. The full path is /Applications/Unity/Unity.app/Contents/Resources, hit Command-Shift-G, paste this path and hit Open. This should get you into the folder (if you haven't renamed the Unity folder).

That's probably a permission issue. Either use a text editor that properly asks for admin permissions when needed (e.g. TextMate does this) or try copying the file to the desktop, edit it there and then copy it back.